Identification of cytoplasmic domains within the epithelial Na⁺ channel reactive at the plasma membrane

The activity of membrane proteins is controlled, in part, by protein-protein interactions localized to the plasma membrane. In the current study, domains within the epithelial Na⁺ channel (ENaC) reactive at the plasma membrane were identified using a novel yeast one-hybrid screen. The cytosolic N terminus of αENaC and the cytosolic C termini of α-, β-, and γENaC contained domains reactive at the plasma membrane. Fluorescent micrographs of epithelial cells overexpressing fusion proteins of enhanced green fluorescent protein and mENaC cytosolic domains were consistent with those in yeast. A novel membrane reactive domain within the cytosolic C terminus of γ-mENaC was localized to the 17 amino acids between residues Thr⁵⁸⁴-Pro⁶⁰⁰. Two overlapping internalization signals within the C terminus of γ-mENaC, a WW-binding domain (PY motif) and a tyrosine-based endocytic signal, were additive with respect to decreasing complementation and expression levels of hybrid proteins. Decreases in expression levels of hybrid proteins containing the PY and endocytic motif were reversed with latrunculin A, an inhibitor of endosomal movement. Decreases in complementation and expression levels of hybrid proteins mediated by the combined PY and overlapping endocytic motif proceeded in the absence of established ubiquitination sites within ENaC. In addition, the endocytic motif was active in the absence of the PY motif, demonstrating that these two domains, while possibly interacting, also have discrete functions. The novel domains within the cytosolic N terminus of aENaC and the C termini of α-, β-, and γENaC identified here are likely to be involved in protein-protein and/or protein-lipid interactions localized to the plasma membrane. We hypothesize that these newly identified domains play a role in modulating ENaC activity.